This invention relates in general to electrophotography and more specifically, to color imaging members and methods of making and using the imaging members.
Generally, electophotographic imaging processes involve the formation and development of electrostatic latent images on the imaging surface of an electrophotographic imaging member. The electrophotographic imaging member is usually imaged by uniformly electrostatically charging the imaging surface in the dark and exposing the member to a pattern of activating electromagnetic radiation such as light to selectively dissipate the charge in the illuminated areas of the member to form an electrostatic latent image on the imaging member surface. The electrostatic latent image is then developed with a developer composition containing toner particles which are attracted to the photoconductive member in image configuration. The resulting toner image is often transferred to a suitable receiving member such as paper and fixed thereto by any suitable technique such as thermal or pressure fusing. This imaging process may be repeated many times with reusable electrophotographic imaging members.
The electrophotographic imaging members include single or multiple layered devices comprising homogenous or heterogenous inorganic or organic compositions and the like. There have been disclosed layered photoreceptor devices comprising photogenerating layers and charge transport layers deposited on conductive substrates as described, for example, in U.S. Pat. No. 4,265,990. Electrophotographic devices known in the art also comprise, for example, a conductive substrate having deposited thereon a single layer comprising an organic photoconductor such as a polyvinylcarbazole-2,4,7-trifluorenone combination, phthalocyanines, quinacridones, pyrazolones and the like. These electrophotographic imaging members all contain at least one electrophotographic insulating material which is electrically insulating in the dark, but electrically conductive when struck by activating radiation.
Photoreceptor devices for color electrophotographic applications are well known and described, for example, in "Imaging Processes and Materials", Neblette's eight edition, Ed. J. Sturge, V. Walworth, A. Shepp, Van Nostrand Reinhold, New York, 1989, Chapter 5, Electrophotography, page 162. In one application for reproducing color images the photoreceptor is sensitive to electromagnetic radiation over the entire visible spectrum. The input color image is separated into three primary colors by appropriate external filters. This process involves three entirely separate latent image forming steps whereby the photoreceptor is sequentially exposed three times to the input image through typically three external color filters and developed stepwise by one of three color toners prior to transfer to paper. Exposure, development and transfer steps typically all occur for one colored toner prior to the next exposure. Three separate development systems, with the transfer of three colored toners are required to provide a full gamut of color. Disadvantages of this multicolor imaging process include the multiple light exposures required for forming the latent color image; the need for using external filters to control the spectral sensitivity of the photoreceptor device; the need for precise registration of the developer housings with respect to the latent image on the photoreceptor; and the need for precise registration of the developed toner patterns when they are transferred to the copy receiving sheet. The multiple light exposures required for latent image formation in the multicolor process have the additional disadvantages of consuming extra energy that powers the illuminant source and is time consuming, making the process slow compared to a single pass exposure process. Speed is also adversely affected if multiple development and transfer passess are required.
In another color photoreceptor application known as "highlight" or "accent" color, multilayer photoreceptor structures have been designed for single pass, two-color electrophotography, for example a black toner on white paper with a second highlight color toner according to Ishida et al as described in "Two Color Electrophotography", 4th International Conference on Electrophotography, Washington, D.C., p. 82. The Ishida et al photoreceptor consists of a conductive substrate, a lower photoconductive layer, an insulating layer in the middle and a photoconductive layer on top. The bottom and top photoconductive layers are chosen so that the lower layer is sensitive to light of wavelength less than 600 nm, while the upper layer is sensitive to wavelengths greater than 600 nm. The lower layer is sensitive only in the blue region of the spectrum. The middle layer may be an insulating polymer. The upper layer may be a polymeric organic photoconductor containing a dye that transmits blue and is sensitive in the red region of the spectrum. The photosensitization is a three step process involving sequential positive charging, negative charging in the dark and image exposure. The image is developed sequentially with a negatively charged colored toner followed by a black toner of opposite polarity. A problem with this two color process is cross contamination of the black toner by the colored toner leading to progressive deterioration of the black toner image quality in subsequently formed black toner images.
The application of dyestuffs in photosensitive members is known in the art for both single color and multi-color electrophotographic imaging processes. The dye or pigment in the photosensitive members of the prior art must serve to sensitize the photosensitive member, that is to promote and sustain the ionic charged state. For example copper phthalocyanine can be used as a photoconductor when incorporated into a polymeric binder to render the photoconductor photosensitive. Dyes have been used as sensitizers even in inorganic photoconductor materials to sensitize the xerographic plate to a larger segment of the visible spectrum. Many of the sensitizers used in photography are also applicable to xerography. Thus for example, crystal violet derivatives and cyanines are also useful sensitizers for electrophotographic imaging members. Organic pigments have also been vacuum deposited to form a photogenerating layer.